1. Field of the Invention
The present invention relates generally to spread spectrum communication systems and, more particularly, is concerned with a method and system for nonlinearizing spread spectrum codes for enhanced system security while still maintaining rapid acquisition thereof.
2. Description of the Prior Art
The present invention constitutes an improvement upon the system described and illustrated in the above cross-referenced U.S. patent application.
The system of the referenced application utilizes, by way of example, a plurality of linear PN component codes, C.sub.1, C.sub.2, . . . C.sub.n, which are all relatively prime with respect to each other, have an approximately equal number of binary ONES and ZEROES, and, with respect to linear MAJ and MOD composites thereof, possess the following correlation properties:
(1) C.sub.1, C.sub.2, . . . and C.sub.n each correlates with MAJ (C.sub.1, C.sub.2, . . . C.sub.n) fifty percent of the time. PA1 (2) C.sub.1, C.sub.2, . . . and C.sub.n each does not correlate with MOD (C.sub.1, C.sub.2, . . . C.sub.n). PA1 (3) MAJ (C.sub.1, C.sub.2, . . . C.sub.n) correlates with MOD (C.sub.1, C.sub.2, . . . C.sub.n) fifty percent of the time. PA1 (1) Linear component codes, C.sub.1, C.sub.2, . . . C.sub.n, are generated. PA1 (2) C.sub.1, C.sub.2, . . . C.sub.n are combined in accordance with the Boolean majority voting rule to form a linear acquisition composite code, MAJ (C.sub.1, C.sub.2, . . . C.sub.n). PA1 (3) The acquisition composite code is transmitted. PA1 (4) Upon receipt of the acquisition composite code, linear reference component codes, R.sub.1, R.sub.2, . . . R.sub.n which correlate respectively with C.sub.1, C.sub.2, . . . C.sub.n, are generated. PA1 (5) First, R.sub.1 is correlated with MAJ (C.sub.1, C.sub.2, . . . C.sub.n); then R.sub.2 is correlated with MAJ (C.sub.1, C.sub.2, . . . C.sub.n ; finally, R.sub.n is correlated with MAJ (C.sub.1, C.sub.2, . . . C.sub.n). PA1 (1) Linear reference component codes, R.sub.1, R.sub.2, . . . R.sub.n, are combined in accordance with the modulo-2 addition rule to form a linear reference composite code, MOD (R.sub.1, R.sub.2, . . . R.sub.n). PA1 (2) MOD (R.sub.1, R.sub.2, . . . R.sub.n) is then correlated with acquisition composite code MAJ (C.sub.1, C.sub.2, . . . C.sub.n). PA1 (3) Linear composite codes, C.sub.1, C.sub.2, . . . C.sub.n, are then combined in accordance with the modulo-2 addition rule to form a linear data-carrying composite code, MOD (C.sub.1, C.sub.2, . . . C.sub.n). PA1 (4) The data-carrying composite code is then transmitted, instead of the acquisition composite code. PA1 (5) At the receiver, MOD (R.sub.1, R.sub.2, . . . R.sub.n) now correlates with MOD (C.sub.1, C.sub.2, . . . C.sub.n).
MAJ (C.sub.1, C.sub.2, . . . C.sub.n) equals a Boolean majority vote of C.sub.1, C.sub.2, . . . and C.sub.n. PA0 MOD (C.sub.1, C.sub.2, . . . C.sub.n) equals a modulo-2 addition of C.sub.1, C.sub.2, . . . and C.sub.n.
The chronological sequence of events carried out for achieving rapid code acquisition by the prior art system may be summarized as follows:
For the transmission and receipt of data, the following chronological sequence of events is carried out by the prior art system:
The above-described sequence of events implies that the total number of code bits required to be searched for acquisition of the transmitted composite code is equal to the sum of the individual lengths of the component codes which form the composite code, rather than the product of their lengths. Consequently, it is readily appreciated that acquisition under the prior art system is rapid, thereby leaving little time for an intelligent jammer to analyze the linear composite code MAJ (C.sub.1, C.sub.2, . . . C.sub.n), which is transmitted for acquisition, in order to determine component codes, C.sub.1, C.sub.2, . . . C.sub.n.
Also, the jammer must have knowledge of all of the component codes and their phase relationship with respect to each other in order to jam MOD (C.sub.1, C.sub.2, . . . C.sub.n) which is used for data transmission, since none of the component codes correlate with MOD (C.sub.1, C.sub.2, . . . C.sub.n).
However, under field conditions where the jammer is capable of intercepting the transmission of MOD (C.sub.1, C.sub.2, . . . C.sub.n), the latter is vulnerable to discovery through analysis by the jammer since it is a linear sequence. By using a computer to perform well known mathmatical calculations at high speed, the polynomial equation which mathematically represents the intercepted linear composite code can be determined and a replica thereof constructed. Therefore, while the overall sequence of events carried out by the prior art system increases the difficulty of code analysis by an enemy, it does not preclude such analysis under certain field conditions in view of the fact that the component codes and the composites thereof being utilized by the prior art system are all linear sequences.